Air traffic control advises the pilot that taxiing to the gate (or to another location where passengers can be deplaned) would significantly disrupt airport operations.

U.S. airlines operating international flights to or from most U.S.airports
must each establish and comply with their own limit on the length of tarmac
delays on those flights. On both domestic and international flights, U.S.
airlines must provide passengers with food and water no later than two hours
after the tarmac delay begins. While the aircraft remains on the tarmac lavatories
must remain operable and medical attention must be available if needed.

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